At the end of the last century, astronomers discovered a startling fact. The Universe has been expanding at a faster pace than before, in the last two billion years or exploding for the second time after the Big Bang! Those who thought that the Universe was slowing down were told that is was actually speeding up!
Astronomers began to suspect that there must be some sort of anti-gravity, dark energy—to counter the usually understood gravitation pulling the Universe down. Some of the astronomers think that 73 per cent of the total mass of the Universe is accounted for by dark energy, 23 per cent is the so-called dark matter, while only 4 per cent represents the matter present in all visible objects. In the 1940’s, astrophysicist Fritz Zwicky, thought that clusters of galaxies consisted predominantly of matter in some non-luminous form. Today he no longer sounds eccentric! It is now believed that when the Universe was almost half its present age, most galaxy formation was downsized and dark energy began to dominate!
Recent observations reveal that the galaxy clusters had not grown much in the past eight billion years. Perhaps, but for dark energy, acceleration of the galaxies would have prevented the formation of stars and eventual emergence of planets and life. On the contrary, if dark energy is dominant, then even the Earth may be snatched away from the Sun! Luckily, that will be a long, long time from now!
We now see many galaxies as they were only 800 million years after the Big Bang. Infrared detectors may show us a still older cosmic scene. It is now known that the star formation was at the maximum at about half the age of the Universe. X-ray, radio and gamma ray detectors may push the limits of the early Universe “visible” now and probe the first generation of stars. They may probably be quite different from those born recently, composed entirely of hydrogen and helium with a trace of lithium, but without the heavier elements that we see in younger stars. We cannot observe the first stars, which are dead and gone in supernovae, leaving heavy elements. But what accounts for the accelerated expansion at the earliest moment of the Universe and what causes its recent acceleration?
A Russian astrophysical laboratory, expected to be launched in 2011, is designed to discover about three million cores of active galaxies (super massive black holes) and up to 100,000 galactic clusters. The observatory will have X-ray and gamma ray detectors. Its unique feature is the low orbit: 580 to 600 km from the Earth, with zero inclination to the equator and below the Van Allen radiation belts. This would help catch the low surface brightness of the celestial objects. The probe is expected to explain the nature of the dark matter and energy.
The most surprising discovery of recent years is the accelerated expansion of the Universe. The dominant form of energy it contains has negative pressure and seems to counter gravity. The ordinary form of matter accounts only for 4 per cent, while the nature of the other 96 per cent (about 73 per cent dark energy and 23 per cent dark matter), is essentially unknown. Scientists are of the view that further progress in understanding the Universe calls for knowledge beyond the ordinary matter and radiation.
For instance, neutrino and gravitational wave astronomy are expected to reveal new phenomena. Basically, theoretical tools are also required even as instruments improve in their efficiency. The dark energy emits no radiation. It interacts with matter only very weakly, it cannot be produced in the laboratory. Present theory of gravity fails at cosmological distances. The quest for dark matter and dark energy continues. The nature of what is unknown is itself not known!
In the history of science, the telescope marks a watershed: until the telescope revealed new celestial objects and their features, science was based on authority and ‘given truths’, not open to question. After the advent of the telescope, science became a discipline based on observation and experiment, open to constant review, irrespective of authority. An open mind on all matters, seen and unseen, emerged as the essence of a scientific approach.
In another profound impact, the telescope has redefined our place in the Universe. It has shown how our solar system is but a speck of dust in the vast Universe, with its billions and billions of stars and unseen matter. Perhaps the most profound discovery of the telescope is the (tentative) measure of the unknown. All our knowledge seems to be limited to less than 4 per cent of what is believed to be the total matter, and the rest is considered hidden.
Searching and understanding the unknown is a continuous process of vision and revision, based on a mastery of basic sciences. In this adventure, the telescope will continue to be a great device and India is well placed, geographically and intellectually, to use it effectively. India, with its rich legacy of astronomical observations, will continue to play a key role in the discovery of our Universe with an open mind in the true scientific spirit of Galileo, celebrated in the International Year of Astronomy.